Wire and arc additive manufacturing method for titanium alloy

ABSTRACT

The present disclosure provides a wire and arc additive manufacturing (WAAM) method for a titanium alloy. The method includes the following steps: step 1: performing a WAAM process assisted by cooling and rolling; step 2: milling side and top surfaces of an additive part; step 3: performing, by friction stir processing (FSP) equipment, an FSP process on the additive part, and applying cooling and rolling to a side wall of the additive part through a cooling and rolling device during the FSP process; step 4: finish-milling the top surface of the additive part for a WAAM process in the next step; and step 5: repeating the above steps cyclically until final forming of the part is finished. This WAAM method completely breaks dendritic structures and refines grains in the WAAM process of the titanium alloy, thereby effectively repairing defects such as pores and cracks.

The present application claims priority to the Chinese PatentApplication No. 201910079333.0, filed with China National IntellectualProperty Administration (CNIPA) on Jan. 28, 2019, and entitled “WIRE ANDARC ADDITIVE MANUFACTURING METHOD FOR TITANIUM ALLOY”, which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure belongs to the technical field of metal additivemanufacturing (AM), and relates to a wire and arc additive manufacturing(WAAM) method for a titanium alloy, in particular to a WAAM methodassisted by cooling and rolling and friction stir processing (FSP) for atitanium alloy.

BACKGROUND

Wire and arc additive manufacturing (WAAM) is an advanced manufacturingtechnology used to build up a three-dimensional (3D) metal part layer bylayer according to a 3D digital model under program control by adding ametal wire according to the principle of discrete deposition. Dependingon the nature of the heat source, there are commonly three types of WAAMprocesses: gas metal arc welding (GMAW)-based, gas tungsten arc welding(GTAW)-based and plasma arc welding (PAW)-based. Compared with additivemanufacturing (AM) that uses laser and electron beams as heat sources,WAAM has the following advantages. 1) WAAM has high deposition rate,high wire utilization and low manufacturing cost. 2) WAAM can formmaterials with high laser reflectivity, such as aluminum alloy. 3) WAAMis not limited by the size of equipment such as forming cylinder andvacuum chamber, and is easy to manufacture large-scale components.

Titanium alloy features low density, high specific strength and specificstiffness, good corrosion resistance, good high temperature mechanicalproperties such as fatigue and creep resistance, and is increasinglyused in aerospace, shipbuilding and weapon manufacturing, etc. However,since titanium alloy has high melting point, easy oxidation, low thermalconductivity and high chemical activity, it is difficult to preparetitanium alloy structural parts by traditional casting and forgingmethods. Therefore, the use of the WAAM technology with high formingefficiency, low manufacturing cost and flexible manufacturing form toprepare titanium alloy structural parts has important practicalsignificance. However, there are some problems needing to be solvedurgently for the WAAM of the titanium alloy.

(1) Forming Accuracy Control (Shape Control)

Forming accuracy is mainly measured by two indicators: geometrical(dimensional) accuracy and surface roughness. During the additiveprocess, multiple thermal cycles will generate high thermal stresses,which will cause deformations of the formed part and base plate,bringing difficulties to the control of dimensional accuracy. Inaddition, due to the serious heat accumulation and poor heat dissipationduring the WAAM process, it is easy to cause collapse and “sagging” atthe joint between layers, and the formed surface is prone to unevenness,which will eventually lead to an increase in the surface roughness ofthe formed part.

(2) Microstructure Control (Performance Control)

1) In the WAAM process, the formed part is easy to have coarse columnarcrystals and segregation-induced inhomogeneous chemical composition,which will further cause property deteriorations, such as grain boundarybrittleness and intergranular corrosion (IGC).

2) In the WAAM process, defects such as pores and thermally inducedcracks are easy to appear, which will reduce the density and corrosionresistance of the deposited metal, reduce the effective bearing area ofthe additive part, and easily cause stress concentration, therebyreducing the strength and plasticity of the additive part.

As common problems faced by the WAAM technology, shape and performancecontrol are also the current research hotspot in this field. At present,in terms of shape control, the problem of sagging and collapse is mainlysolved by heat input control means such as welding process parameteroptimization and path planning, and the problem of rough surface of theformed part is mainly solved by finishing after forming or applyingother methods during the forming process. In terms of performancecontrol, in recent years, domestic and foreign scholars have proposedthe use of forging, rolling, ultrasonic oscillation and other methods inthe WAAM process to eliminate pores, break dendrites and refine theformed microstructure, so as to achieve the purpose of improvedproperties. Huazhong University of Science and Technology (HUST) inChina and Cranfield University in the United Kingdom have proposedmethods to refine the microstructure of the deposited layer by rollingdeformation in the WAAM process. The rolling deformation proposed byHUST is closer to “in-situ rolling”, that is, a specially designed rollor extrusion device is located directly behind the welding gun and moveswith the welding gun (Haiou, Z., Xiangping, W., Guilan, W., & Yang, Z.Hybrid direct manufacturing method of metallic parts using depositionand micro continuous rolling. Rapid Prototyping Journal, 2013, 19(6):387-394). The method proposed by Cranfield University is “interlayerrolling”, that is, rolling is applied after one or several layersdeposited by arc welding are cooled to ambient temperature (Colegrove, PA, Coules, H E, Fairman, J., Martina, F., Kashoob, T., Mamash, H., &Cozzolino, L D. Microstructure and residual stress improvement in wireand arc additively manufactured parts through high-pressure rolling.Journal of Materials Processing Technology, 2013, 213(10): 1782-1791).The rolling method reduces the porosity, and enables staticrecrystallization (SRX) in the treatment zone of the formed part. Due tothe SRX, the grain size is reduced to less than 30 μm, of which the 0-5μm grains account for about 49%, the 5-10 μm grains account for about30%, and the 10-30 μm grains account for the rest. The effect of“in-situ rolling” and “interlayer rolling” in improving thesolidification structure is related to the strain applied during theforming process. In other words, a greater rolling force applied leadsto a denser and finer structure. But this also leads to obviouslimitations in the forming of complex thin-walled components.

Chinese patent CN106735967A discloses a shape and performance controlmethod for an ultrasonic vibration assisted WAAM process. During theWAAM process, this method applies non-contact ultrasonic vibration tothe molten pool synchronously to break crystal grains in the molten pooland inhibit the growth of the crystal grains, so as to refine thegrains. However, this method does not solve the problems of pores andgrain boundary liquefaction occurring in the WAAM process.

Friction stir processing (FSP) is a technology developed on the basis offriction stir welding (FSW) for material microstructure modification andpreparation of new materials. Similar to FSW, in FSP, a high-speedrotating pin of the tool is inserted into the material, and the strongstirring motion of the pin causes the material of the workpiece toundergo violent plastic deformation, mixing and break. In this way, theFSP process realizes the densification, homogenization and refinement ofthe material microstructure and improves the properties of the material.FSP has achieved good results in the preparation offine-grain/ultra-fine-grain materials and surface/bulk compositematerials, microstructure modification of heterogeneous materials, andlocal hardening/defect repairs of workpieces. Specifically, FSP has thefollowing advantages. (1) Refine grains and improve material properties.In the FSP process, under the combined conditions of large strain andhigh temperature, uniformly refined equiaxed grains are generated in thestir zone through dynamic recrystallization (DRX), which improves themechanical properties of the material. (2) Eliminate defects and obtaina uniform and dense microstructure of the material. The FSP can breakcoarse second phase particles and aluminum dendrites of the castaluminum alloy, eliminate casting pores and refine matrix grains,thereby significantly improving the mechanical properties of thematerial, especially plasticity and fatigue properties. By using the FSPtechnology to modify the hypereutectic A390 aluminum-silicon alloy, T.S. Mahmoud found that the FSP technology can reduce casting shrinkageporosity and has obvious refining effect on α-Al and Si particles(Mahmoud T S. Surface modification of A390 hypereutectic Al—Si castalloys using friction stir processing. Surface & Coatings Technology,2013, 228(9): 209-220). In addition, a report has shown that FSPtreatment performed in the weld of a fusion welded joint can cause DRXto eliminate welding defects such as dendrite segregation, pores andthermally induced cracks, thereby improving the overall properties ofthe joint (G K Padhy. Friction stir based welding and processingtechnologies-processes, parameters, microstructures and applications: Areview. Journal of Materials Science & Technology, 2018, 34(9):1-38).(3) Reduce structural residual stress. As FSP is a solid-stateprocessing technology with low heat input, the thermal deformation andresidual stress of the material after processing are small.

In summary, the combination of interlayer FSP modification in the WAAMprocess of titanium alloy helps to refine the internal microstructure ofthe additive part, eliminate defects such as pores and cracks, andachieve a dense microstructure with uniform chemical composition. Inaddition, it can reduce thermal deformation, reduce residual stress, andimprove the mechanical properties of the additive part. However, whenthe FSP method is used to modify the additive part, the stir zone of thestirring pin is limited, which makes it difficult to process the metalon the side wall of the additive part, so the side wall metal stillretains the cast microstructure. In addition, in the subsequent WAAMprocess or FSP modification, the microstructure of the deposited metalmodified by FSP in the previous layer will be coarsened due to multiplethermal cycles, resulting in a decrease in the properties of theadditive part.

SUMMARY

In order to solve the problems existing in the prior art, the presentdisclosure provides a wire and arc additive manufacturing (WAAM) methodfor a titanium alloy.

A WAAM method for a titanium alloy, including the following steps:

step 1: drawing a part model through three-dimensional (3D) drawingsoftware; performing layered slicing on the part model through slicingsoftware to acquire layered slice data; simulating the layered slicedata through simulation software, and optimizing a forming path togenerate a robot control code; importing the robot control code into awelding robot; performing, by the welding robot, a WAAM process with atitanium alloy on a pre-prepared base plate to form a multi-layerdeposited metal including a total of 2 to 4 layers; applying, during theforming process, cooling and rolling to a side wall of the multi-layerdeposited metal through a cooling and rolling device, where duringcooling and rolling, 10-40° C. cooling water flows at a rate of1,000-3,000 L/h, and a rolling stress on the multi-layer deposited metalis 50-400 MPa;

step 2: milling side and top surfaces of the multi-layer depositedmetal;

step 3: performing, by an FSP device, an FSP process on the multi-layerdeposited metal after milling, and applying cooling and rolling to theside wall of the multi-layer deposited metal through the cooling androlling device during the FSP process, where during cooling and rolling,10-40° C. cooling water flows at a rate of 1,000-3,000 L/h, and arolling stress on the multi-layer deposited metal is 100-800 MPa;

step 4: finish-milling the top surface of the multi-layer depositedmetal to make the treatment surface smooth for a WAAM process in thenext step; and

step 5: repeating the above steps cyclically until the multi-layerdeposited metal is formed into an additive part with a preset shape andsize, where

the cooling and rolling device includes a roller, a heat conductingcylinder and a heat conducting outer ring; the heat conducting outerring is rotatably assembled on an outer wall of the heat conductingcylinder; the heat conducting cylinder is provided with an inner cavity;an upper surface of the heat conducting cylinder is provided with acooling water inlet communicating with the inner cavity; a lower surfaceof the heat conducting cylinder is provided with a cooling water outletcommunicating with the inner cavity; the roller is vertically fixed andassembled at a center of the upper surface of the heat conductingcylinder, and the roller moves synchronously with a welding gun of thewelding robot or a stirring tool of the FSP device.

In step 1, the WAAM process is performed according to parametersincluding: welding current 66-300 A, welding voltage 15.0-25.0 V, wireswing amplitude 2.0-6.5 mm, wire swing speed 600-1,600 mm/min, formingspeed 100-400 mm/min, and lifting height of the welding gun at eachlayer 1.0-2.5 mm.

In step 1, the multi-layer deposited metal formed by the WAAM process is7-50 mm in width.

In step 1, the multi-layer deposited metal is formed by single-passmulti-layer deposition or multi-pass multi-layer deposition.

In step 2, a milling rate on the side and top surfaces of themulti-layer deposited metal is 0.1-0.5 mm and 0.3-2.2 mm, respectively.

In step 3, a length of a pin of the stirring tool of the FSP device isgreater than a height of the multi-layer deposited metal after milling,and a diameter of a shoulder of the stirring tool is slightly smallerthan a width of the multi-layer deposited metal after milling.

The FSP device adopts parameters including: shoulder diameter of thestirring tool 6-46 mm, pin length 2-5 mm, rotation speed of the stirringtool 800-2,000 r/min, traveling speed 40-200 mm/min, and inclinationangle of the stirring tool 1.5-3°.

The present disclosure provides a WAAM method assisted by cooling androlling and FSP for a titanium alloy. In the cooling and rollingassisted WAAM process, the formed multi-layer deposited metal is subjectto cooling and rolling and FSP modification after every 2 to 4 layers oftitanium alloy are deposited.

In the cooling and rolling assisted WAAM process, the cooling androlling device cools and rolls the side wall of the multi-layerdeposited metal while the titanium alloy is deposited layer by layer byan arc. This helps to reduce a thermal impact of the deposition processon a previous layer of metal, and can control geometrical dimensions ofthe multi-layer deposited metal through the effect of rolling. In theprocess of cooling and rolling and FSP modification of the depositedmetal, the cooling and rolling device and the shoulder of the stirringtool form a partial cavity to ensure that the multi-layer depositedmetal surrounded by the cavity has a defect-free forged microstructureformed under the action of the cooling and rolling device and thestirring tool. Meanwhile, a cooler on the cooling and rolling deviceavoids coarsening of the microstructure due to overheating of a FSP stirzone and the previous layer of metal. In this way, the multi-layerdeposited metal is formed with an excellent microstructure, and themechanical properties of the multi-layer deposited metal are improved.In addition, the problems of poor dimensional accuracy and roughness ofthe additive part are prevented.

Compared with the prior art, the present disclosure has significantadvantages in improving the dimensional accuracy of the component andreducing the roughness of the component. Above all, in the WAAM processof the titanium alloy, the present disclosure completely destroysdendritic growth and refines grains, thereby effectively repairingdefects such as pores and cracks. In addition, in the WAAM andmodification process, the present disclosure prevents, by applyingcooling, overheating of the multi-layer deposited metal and coarseningof the microstructure caused thereby, thus greatly improving themechanical properties of the multi-layer deposited metal, especiallyplasticity and fatigue properties. The cooling and rolling device of thepresent disclosure has high flexibility and is suited for WAAM ofstraight-walled structural parts and curved structural parts.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a wire and arc additive manufacturing(WAAM) process assisted by cooling and rolling according to the presentdisclosure.

FIG. 2 is a sectional view of a cooling and rolling device according tothe present disclosure.

FIG. 3 is a schematic view of milling side and top surfaces of amulti-layer deposited metal according to the present disclosure.

FIG. 4 is a schematic view of friction stir processing (FSP) assisted bycooling and rolling according to the present disclosure.

Reference Numerals: 1. base plate; 2. multi-layer deposited metal; 3.cooling and rolling device; 31. roller; 32. heat conducting cylinder;33. heat conducting outer ring; 34. inner cavity; 35. cooling waterinlet; 36. cooling water outlet; 37. ball; 4. stirring tool; 5. weldinggun; and 6. milling cutter.

DETAILED DESCRIPTION

The present disclosure is further described below with reference to theembodiments and accompanying drawings.

It should be noted that all directional indications (such as upper,lower, left, right, front and back) in the embodiments of the presentdisclosure are merely used to explain a relative position relationshipor motions of components in a specific gesture (as shown in thedrawings). If the specific gesture changes, the directional indicationswill change accordingly.

Embodiment 1

As shown in FIGS. 1 to 4, the present disclosure provides a method forfabricating a 01.6 mm Ti-6Al-4V straight-walled part by wire and arcadditive manufacturing (WAAM). The method includes the following steps:

Step 1: A cooling and rolling-assisted WAAM process is performed. Amodel of a 300 mm (length)×100 mm (height)×13 mm (width) straight-walledpart is drawn through three-dimensional (3D) drawing software. Layeredslicing is performed on the part model through slicing software toacquire layered slice data. The layered slice data is simulated throughsimulation software, and a forming path is optimized to generate a robotcontrol code (or a digital control code). The robot control code isimported into a welding robot. A WAAM process is performed by thewelding robot on a pre-prepared T-shaped base plate 1 by using an arcgenerated by a tungsten inert gas (TIG) welder as a heat source to forma multi-layer deposited metal 2 including a total of 2 to 4 layers ofTi-6Al-4V. The multi-layer deposited metal 2 is 13 mm in width, and themulti-layer deposited metal 2 is formed by single-pass multi-layerdeposition. The WAAM process is performed according to parametersincluding: welding current 110 A, welding voltage 21.5 V, forming speed250 mm/min, wire feed speed 2.5 m/min, lifting height of a welding gunat each layer 2 mm, wire swing amplitude 2.75 mm, interlayer interval 3min and wire swing speed 800 mm/min.

Meanwhile, cooling and rolling is applied to a side wall of themulti-layer deposited metal 2 through a cooling and rolling device 3during the forming process. The cooling and rolling applied to the sidewall of the multi-layer deposited metal 2 by the cooling and rollingdevice 3 helps to reduce a thermal impact of the deposition process on aprevious layer of metal, and can control geometrical dimensions of themulti-layer deposited metal 2 through the effect of rolling. Duringcooling and rolling, 10° C. cooling water flows at a rate of 1,500 L/h,and a rolling stress on the multi-layer deposited metal is 150 MPa.

Step 2: Side and top surfaces of the multi-layer deposited metal 2 aremilled by a milling robot by using a milling cutter 6. This step aims tocontrol the dimensional accuracy of the multi-layer deposited metal 2and provide a smooth surface for subsequent friction stir processing(FSP) so as to prevent defects occurring in the FSP process. Two sidesurfaces of the multi-layer deposited metal 2 are milled with a millingrate of 0.3 mm, and a top surface of the multi-layer deposited metal ismilled with a milling rate of 2 mm. During the milling process, themilling rate, feed speed and other process parameters are set accordingto a dimensional accuracy required by a final part.

Step 3: An FSP process is performed by an FSP device on the multi-layerdeposited metal 2 after milling. A length of a pin of a stirring tool 4of the FSP device is greater than a height of the multi-layer depositedmetal 2 after milling, and a diameter of a shoulder of the stirring tool4 is slightly smaller than a width of the multi-layer deposited metal 2after milling. Thus, the microstructure of the multi-layer depositedmetal 2 can be refined to the greatest extent so as to eliminatedefects. Specifically, the FSP device adopts parameters including:shoulder diameter of the stirring tool 4: 12.5 mm; pin length: 5 mm;rotation speed of the stirring tool 4: 800 r/min; traveling speed: 60mm/min; and inclination angle of the stirring tool 4: 2.5°.

Meanwhile, cooling and rolling is applied to the side wall of themulti-layer deposited metal 2 through the cooling and rolling device 3during the FSP process. The cooling and rolling device and the shoulderof the stirring tool 4 form a partial cavity to ensure that themulti-layer deposited metal 2 surrounded by the cavity has a defect-freeforged microstructure formed under the action of the cooling and rollingdevice and the stirring tool 4. Meanwhile, the cooling water filled inthe cooling and rolling device 3 avoids coarsening of the microstructuredue to overheating of a FSP stir zone and the previous layer of metal.During cooling and rolling, 15° C. cooling water flows at a rate of1,800 L/h, and a rolling stress on the multi-layer deposited metal is300 MPa.

Step 4: The top surface of the multi-layer deposited metal 2 isfinish-milled at a milling rate of 0.3 mm by the milling cutter 6 of themilling robot or a milling machine tool to make the treatment surfacesmooth for a WAAM process in the next step.

Step 5: The above steps are repeated cyclically until the multi-layerdeposited metal 2 is formed into an additive part with a preset shapeand size.

The cooling and rolling device 3 includes a roller 31, a heat conductingcylinder 32 and a heat conducting outer ring 33. The heat conductingouter ring 33 is rotatably assembled on an outer wall of the heatconducting cylinder 32 through a ball 37. The heat conducting cylinder32 is provided with an inner cavity 34. An upper surface of the heatconducting cylinder 32 is provided with a cooling water inlet 35communicating with the inner cavity 34; a lower surface of the heatconducting cylinder 32 is provided with a cooling water outlet 36communicating with the inner cavity 34. The roller 31 is verticallyfixed and assembled at a center of the upper surface of the heatconducting cylinder 32, and the roller 31 moves synchronously with awelding gun 5 of the welding robot or a stirring tool 4 of the FSPdevice.

The heat conducting cylinder 32 and the heat conducting outer ring 33may be made of a metal material with good heat conductivity.

The present disclosure ensures that the microstructure in the stir zonetreated by the cooling and rolling and FSP is composed of fine equiaxedgrains, and eliminates defects such as pores, holes and liquefactioncracks that are easily generated in an ordinary WAAM process, therebyimproving the mechanical properties of the formed part. Table 1 showsdata of mechanical properties of Ti-6Al-4V thin-walled parts formed by aWAAM process assisted by cooling and rolling and FSP, an ordinary WAAMprocess and a casting process.

TABLE 1 Comparison of mechanical properties Parallel to the weldingdirection (X) Yield Tensile strength strength Elongation σ_(0.2)/MPaσ_(s)/MPa δ/% Ti-6A1-4V thin-walled part formed 888 1009 13 by WAAMprocess assisted by cooling and rolling and FSP Ti-6A1-4V thin-walledpart formed — 911 6.705 by ordinary WAAM process Ti-6A1-4V thin-walledpart formed 800 895 6 by casting process

Embodiment 2

This embodiment provides a method for fabricating a Φ1.6 mm Ti-6Al-4Vstraight-walled part by WAAM. The method includes the following steps:

Step 1: A cooling and rolling-assisted WAAM process is performed. Amodel of a 300 mm (length)×100 mm (height)×42 mm (width) straight-walledpart is drawn through 3D drawing software. Layered slicing is performedon the part model through slicing software to acquire layered slicedata. The layered slice data is simulated through simulation software,and a forming path is optimized to generate a robot control code (or adigital control code). The robot control code is imported into a weldingrobot. A WAAM process is performed by the welding robot on apre-prepared T-shaped base plate 1 by using an arc generated by a TIGwelder as a heat source to form a multi-layer deposited metal 2including a total of 2 to 4 layers of Ti-6Al-4V. The multi-layerdeposited metal 2 is 42 mm in width, and the multi-layer deposited metal2 is formed by multi-pass multi-layer deposition. The WAAM process isperformed according to parameters including: welding current 300 A,welding voltage 25.0V, forming speed 150 mm/min, wire feed speed 2.5m/min, lifting height of a welding gun at each layer 1 mm, wire swingamplitude 2.5 mm, interlayer interval 3 min and wire swing speed 600mm/min.

Meanwhile, cooling and rolling is applied to a side wall of themulti-layer deposited metal 2 through a cooling and rolling device 3during the forming process. The cooling and rolling applied to the sidewall of the multi-layer deposited metal 2 by the cooling and rollingdevice 3 helps to reduce a thermal impact of the deposition process on aprevious layer of metal, and can control geometrical dimensions of themulti-layer deposited metal 2 through the effect of rolling. Duringcooling and rolling, 10° C. cooling water flows at a rate of 800 L/h,and a rolling stress on the multi-layer deposited metal is 380 MPa.

Step 2: Side and top surfaces of the multi-layer deposited metal 2 aremilled by a milling robot by using a milling cutter 6. This step aims tocontrol the dimensional accuracy of the multi-layer deposited metal 2and provide a smooth surface for a subsequent FSP process so as toprevent defects occurring in the FSP process. Two side surfaces of themulti-layer deposited metal 2 are milled with a milling rate of 0.3 mm,and a top surface of the multi-layer deposited metal is milled with amilling rate of 2 mm. During the milling process, the milling rate, feedspeed and other process parameters are set according to a dimensionalaccuracy required by a final part.

Step 3: An FSP process is performed by an FSP device on the multi-layerdeposited metal 2 after milling. A length of a pin of a stirring tool 4of the FSP device is greater than a height of the multi-layer depositedmetal 2 after milling, and a diameter of a shoulder of the stirring tool4 is slightly smaller than a width of the multi-layer deposited metal 2after milling. Thus, the microstructure of the multi-layer depositedmetal 2 can be refined to the greatest extent so as to eliminatedefects. Specifically, the FSP device adopts parameters including:shoulder diameter of the stirring tool 4: 40 mm; pin length: 3 mm;rotation speed of the stirring tool 4: 2,000 r/min; traveling speed: 60mm/min; and inclination angle of the stirring tool 4: 2.5°.

Meanwhile, cooling and rolling is applied to the side wall of themulti-layer deposited metal 2 through the cooling and rolling device 3during the FSP process. The cooling and rolling device and the shoulderof the stirring tool 4 form a partial cavity to ensure that themulti-layer deposited metal 2 surrounded by the cavity has a defect-freeforged microstructure formed under the action of the cooling and rollingdevice and the stirring tool 4. Meanwhile, the cooling water filled inthe cooling and rolling device 3 avoids coarsening of the microstructuredue to overheating of an FSP stir zone and the previous layer of metal.During cooling and rolling, 30° C. cooling water flows at a rate of2,000 L/h, and a rolling stress on the multi-layer deposited metal is750 MPa.

Step 4: The top surface of the multi-layer deposited metal 2 isfinish-milled at a milling rate of 0.3 mm by the milling cutter 6 of themilling robot or a milling machine tool to make the treatment surfacesmooth for a WAAM process in the next step.

Step 5: The above steps are repeated cyclically until the multi-layerdeposited metal 2 is formed into an additive part with a preset shapeand size.

The cooling and rolling device 3 includes a roller 31, a heat conductingcylinder 32 and a heat conducting outer ring 33. The heat conductingouter ring 33 is rotatably assembled on an outer wall of the heatconducting cylinder 32 through a ball 37. The heat conducting cylinder32 is provided with an inner cavity 34. An upper surface of the heatconducting cylinder 32 is provided with a cooling water inlet 35communicating with the inner cavity 34; a lower surface of the heatconducting cylinder 32 is provided with a cooling water outlet 36communicating with the inner cavity 34. The roller 31 is verticallyfixed and assembled at a center of the upper surface of the heatconducting cylinder 32, and the roller 31 moves synchronously with awelding gun 5 of the welding robot or a stirring tool 4 of the FSPdevice.

The heat conducting cylinder 32 and the heat conducting outer ring 33may be made of a metal material with good heat conductivity.

Embodiment 3

This embodiment provides a method for fabricating a 01.6 mm Ti-6Al-4Vstraight-walled part by WAAM. The method includes the following steps:

Step 1: A cooling and rolling-assisted WAAM process is performed. Amodel of a 300 mm (length)×100 mm (height)×25 mm (width) straight-walledpart is drawn through 3D drawing software. Layered slicing is performedon the part model through slicing software to acquire layered slicedata. The layered slice data is simulated through simulation software,and a forming path is optimized to generate a robot control code (or adigital control code). The robot control code is imported into a weldingrobot. A WAAM process is performed by the welding robot on apre-prepared T-shaped base plate 1 by using an arc generated by a TIGwelder as a heat source to form a multi-layer deposited metal 2including a total of 2 to 4 layers of Ti-6Al-4V. The multi-layerdeposited metal 2 is 25 mm in width, and the multi-layer deposited metal2 is formed by multi-pass multi-layer deposition. The WAAM process isperformed according to parameters including: welding current 120 A,welding voltage 23.5V, forming speed 350 mm/min, wire feed speed 2.5m/min, lifting height of a welding gun at each layer 2 mm, wire swingamplitude 4 mm, interlayer interval 3 min and wire swing speed 1,400mm/min.

Meanwhile, cooling and rolling is applied to a side wall of themulti-layer deposited metal 2 through a cooling and rolling device 3during the forming process. The cooling and rolling applied to the sidewall of the multi-layer deposited metal 2 by the cooling and rollingdevice 3 helps to reduce a thermal impact of the deposition process on aprevious layer of metal, and can control geometrical dimensions of themulti-layer deposited metal 2 through the effect of rolling. Duringcooling and rolling, 10° C. cooling water flows at a rate of 1,000 L/h,and a rolling stress on the multi-layer deposited metal is 200 MPa.

Step 2: Side and top surfaces of the multi-layer deposited metal 2 aremilled by a milling robot by using a milling cutter 6. This step aims tocontrol the dimensional accuracy of the multi-layer deposited metal 2and provide a smooth surface for a subsequent FSP process so as toprevent defects occurring in the FSP process. Two side surfaces of themulti-layer deposited metal 2 are milled with a milling rate of 0.3 mm,and a top surface of the multi-layer deposited metal is milled with amilling rate of 2 mm. During the milling process, the milling rate, feedspeed and other process parameters are set according to a dimensionalaccuracy required by a final part.

Step 3: An FSP process is performed by an FSP device on the multi-layerdeposited metal 2 after milling. A length of a pin of a stirring tool 4of the FSP device is greater than a height of the multi-layer depositedmetal 2 after milling, and a diameter of a shoulder of the stirring tool4 is slightly smaller than a width of the multi-layer deposited metal 2after milling. Thus, the microstructure of the multi-layer depositedmetal 2 can be refined to the greatest extent so as to eliminatedefects. Specifically, the FSP device adopts parameters including:shoulder diameter of the stirring tool 4: 24 mm; pin length: 4 mm;rotation speed of the stirring tool 4: 1,600 r/min; traveling speed: 150mm/min; and inclination angle of the stirring tool 4: 1.5°.

Meanwhile, cooling and rolling is applied to the side wall of themulti-layer deposited metal 2 through the cooling and rolling device 3during the FSP process. The cooling and rolling device and the shoulderof the stirring tool 4 form a partial cavity to ensure that themulti-layer deposited metal 2 surrounded by the cavity has a defect-freeforged microstructure formed under the action of the cooling and rollingdevice and the stirring tool 4. Meanwhile, the cooling water filled inthe cooling and rolling device 3 avoids coarsening of the microstructuredue to overheating of an FSP stir zone and the previous layer of metal.During cooling and rolling, 35° C. cooling water flows at a rate of3,000 L/h, and a rolling stress on the multi-layer deposited metal is500 MPa.

Step 4: The top surface of the multi-layer deposited metal 2 isfinish-milled at a milling rate of 0.3 mm by the milling cutter 6 of themilling robot or a milling machine tool to make the treatment surfacesmooth for a WAAM process in the next step.

Step 5: The above steps are repeated cyclically until the multi-layerdeposited metal 2 is formed into an additive part with a preset shapeand size.

The cooling and rolling device 3 includes a roller 31, a heat conductingcylinder 32 and a heat conducting outer ring 33. The heat conductingouter ring 33 is rotatably assembled on an outer wall of the heatconducting cylinder 32 through a ball 37. The heat conducting cylinder32 is provided with an inner cavity 34. An upper surface of the heatconducting cylinder 32 is provided with a cooling water inlet 35communicating with the inner cavity 34; a lower surface of the heatconducting cylinder 32 is provided with a cooling water outlet 36communicating with the inner cavity 34. The roller 31 is verticallyfixed and assembled at a center of the upper surface of the heatconducting cylinder 32, and the roller 31 moves synchronously with awelding gun 5 of the welding robot or a stirring tool 4 of the FSPdevice.

The heat conducting cylinder 32 and the heat conducting outer ring 33may be made of a metal material with good heat conductivity.

By applying cooling through the cooling and rolling device in the WAAMprocess and FSP modification, the present disclosure preventsoverheating of the multi-layer deposited metal and coarsening of themicrostructure caused thereby, thus greatly improving the mechanicalproperties of the multi-layer deposited metal. In addition, the presentdisclosure achieves the purpose of controlling the geometricaldimensions of the multi-layer deposited metal through the cooling androlling device.

The above embodiments are merely intended to illustrate the technicalsolutions of the present disclosure, rather than to make a limitationthereto. Although the present disclosure is described in detail withreference to the above-mentioned embodiments, those of ordinary skill inthe art should understand that similar technical methods may still bederived from the technical solutions described in accordance with theaccompanying drawings and embodiments. If an arc generated by a welderof metal inert gas (MIG), TIG or plasma arc welding (PAW) is used as theheat source and titanium alloy wires with different diameters anddifferent alloy element contents are used as the filler metals, in theWAAM process, the interlayer cooling and rolling and FSP methods can beused to eliminate defects such as pores, liquefaction and cracks of thedeposited metal and refine the microstructure so as to improveproperties. Any simple modifications, equivalent changes andmodifications made to the above embodiments based on the technicalessence of the present disclosure should fall within the scope of thetechnical solutions of the present disclosure.

1. A wire and arc additive manufacturing (WAAM) method for a titaniumalloy, the method comprising the steps: (a) drawing a part model throughthree-dimensional (3D) drawing software; performing layered slicing onthe part model through slicing software to acquire layered slice data;simulating the layered slice data through simulation software togenerate a robot control code; importing the robot control code into awelding robot; performing, by the welding robot, a WAAM process with atitanium alloy on a pre-prepared base plate to form a multi-layerdeposited metal including a total of 2 to 4 layers; applying, during theforming process, cooling and rolling to a side wall of the multi-layerdeposited metal through a cooling and rolling device, wherein duringcooling and rolling, 10-40° C. cooling water flows at a rate of1,000-3,000 L/h, and a rolling stress on the multi-layer deposited metalis 50-400 MPa; (b) milling side and top surfaces of the multi-layerdeposited metal; (c) performing, by friction stir processing (FSP)equipment, an FSP process on the multi-layer deposited metal aftermilling, and applying cooling and rolling to the side wall of themulti-layer deposited metal through the cooling and rolling deviceduring the FSP process, wherein during cooling and rolling, 10-40° C.cooling water flows at a rate of 1,000-3,000 L/h, and a rolling stresson the multi-layer deposited metal is 100-800 MPa; (d) finish-millingthe top surface of the multi-layer deposited metal to make the treatmentsurface smooth for a WAAM process in the next step; and (e) repeatingthe above steps cyclically until the multi-layer deposited metal isformed into an additive part with a preset shape and size, wherein thecooling and rolling device comprises a roller, a heat conductingcylinder and a heat conducting outer ring; the heat conducting outerring is rotatably assembled on an outer wall of the heat conductingcylinder; the heat conducting cylinder is provided with an inner cavity;an upper surface of the heat conducting cylinder is provided with acooling water inlet communicating with the inner cavity; a lower surfaceof the heat conducting cylinder is provided with a cooling water outletcommunicating with the inner cavity; the roller is vertically fixed andassembled at a center of the upper surface of the heat conductingcylinder, and the roller moves synchronously with a welding gun of thewelding robot or a stirring tool of the FSP device.
 2. The WAAM methodfor a titanium alloy according to claim 1, wherein in step (c), a lengthof a pin of the stirring tool of the FSP device is greater than a heightof the multi-layer deposited metal after milling, and a diameter of ashoulder of the stirring tool is slightly smaller than a width of themulti-layer deposited metal after milling.
 3. The WAAM method for atitanium alloy according to claim 1, wherein in step (a), themulti-layer deposited metal formed by the WAAM process is 7-50 mm inwidth.
 4. The WAAM method for a titanium alloy according to claim 1,wherein in step (a), the multi-layer deposited metal is formed bysingle-pass multi-layer deposition or multi-pass multi-layer deposition.5. The WAAM method for a titanium alloy according to claim 1, wherein instep (a), the WAAM process is performed according to parameterscomprising: welding current 60-300 A, welding voltage 15.2-25.0 V, wireswing amplitude 2.1-5.6 mm, wire swing speed 600-1,600 mm/min, formingspeed 140-400 mm/min, and lifting height of the welding gun at eachlayer 0.8-2.1 mm.
 6. The WAAM method for a titanium alloy according toclaim 1, wherein in step (b), a milling rate on the side and topsurfaces of the multi-layer deposited metal is 0.1-0.5 mm and 0.3-2.2mm, respectively.
 7. The WAAM method for a magnesium alloy according toclaim 2, wherein the FSP device adopts parameters comprising: shoulderdiameter of the stirring tool 6-46 mm, pin length 2-5 mm, rotation speedof the stirring tool 800-2,000 r/min, traveling speed 40-200 mm/min, andinclination angle of the stirring tool 1.5-3°.